The present invention relates generally to means for rigidly and hermetically and rigidly sealing together two telescopingly assembled workpieces. It relates particularly to, but is not limited to, such means for establishing a seal between a metal end cap and the end portion of an insulating tubular electrical fuse casing into which the cap is inserted.
Certain electrical devices are especially sensitive to their surroundings and are therefore enclosed in a hermetically sealed housing. For such devices which are then exposed to adverse conditions, such as long term weathering or immersion in hot transformer oil, it is important that the seals of the housing reliably maintain their integrity despite being subjected to numerous temperature cycles. For products which are manufactured in large numbers, and which include seals between parts having significantly different thermal expansion characteristics, such seals are difficult to achieve without requiring costly greater demensional precision of parts.
One electrical device which requires such a rigid hermetic seal, and which is manufactured in large numbers, is an electric current-limiting fuse. Current limiting fuses are used to interrupt large fault currents at high voltages in a controlled manner without in so doing generating damaging voltage transients in the electrical system which they protect.
Typically, a current-limiting fuse has an elongated, tubular insulating housing tube which is closed at both ends by metal terminal end caps. Supported between the end caps, and extending the length of the fuse, is a support core on which are helically wound one or more fusible wire or ribbon elements connected between the end caps. The remaining space in the interior of the housing is filled with a tightly-packed particulate filler of arc-quenching material, such as quartz sand.
When a fault current passes through the fuse element, the element melts, and one or more arcs are generated between the free ends of the severed element. The arcs interact in a controlled manner with the filler, so that as they become elongated with the progressive burning back of the element, their resistance increases greatly until the current is so small that the arcs can no longer be sustained and are finally extinguished to open the circuit.
Contaminants such as moisture or oil inside the fuse can result in its failure either during, or subsequent to its operation by, for example, providing a parallel conductive path to the fusible element or greatly increasing the pressure shock wave due to the release of energy from the vaporization of water or oil. Yet, such fuses are commonly located openly outdoors or immersed in the oil of a transformer. Therefore, the integrity of the seal between the housing tube and the end caps is a critical factor in preventing failure of the fuse.
One present type of seal for fuses is made by interposing a resilient gasket between the housing tube and a metal end cap which fits over the end portion of the tube and then crimping the cap wall tightly against the gasket to form a compression seal. A disadvantage of this approach is that it does not permit the use of end caps which fit inside the tube to make a constant diameter housing. A constant diameter housing is better suited for bayonnet fuse holders and also makes better use of available space inside the fuse ends. Another disadvantage is that the resulting seal is not rigid, the cap being held in place primarily by the friction between the cap to gasket and gasket to tube surfaces. The pressure of gases generated in the fuse when it operates may be sufficient to overcome the frictional forces and push the cap off the end. Still another and more serious disadvantage of such a compression seal is that when the fuse is immersed in transformer oil, the oil can seep under the gasket along minute surface imperfections in the tube, which is typically a glass fiber-reinforced epoxy resin composition.
Another present type of seal is one in which the caps are rigidly attached to the tube by mechanical means, such as pins through the tube and cap, and then the entire end of the fuse jacketed in epoxy resin. It has been found, however, that thermal cycling can result in cracking of the jacket which breaks the seal.
Still another type of seal presently used for fuses is one in which the entire body of the assembled fuse, except the terminals, is jacketed with an epoxy material especially formulated to have a coefficient of thermal expansion closely matching that of the housing tube. Fuses with such a seal are described, for example, in U.S. Pat. No. 3,723,930 issued 27 Mar. 1973 to R. E. Koch and assigned to the same assignee as are the rights to the present invention. While jacketed fuses of this type operate satisfactorily, their manufacturing cost is relatively high due to special procedures required for preparing and applying the epoxy jacket. Lowering the cost of the procedures by automation however, would require too great an investment to be justified by the extent of the total product market.